1. Field of the Invention
The present invention relates to a driving circuit for an electro-luminescence (EL) element.
2. Description of the Related Art
An EL element is used as a back light of a liquid crystal display portion included in a small electronic apparatus such as a wrist watch. There is a need for a small driving circuit for generating an AC voltage for driving this type of EL element. Since a transformer as a voltage boosting component as is generally used in the prior art is disadvantageous in pursuing compactness, a driving circuit as shown in FIG. 4 has been proposed. This circuit is similar to that disclosed in Japanese patent publication No. H8-17114 and comprises an EL element 41 and an inductor 42 connected in series to form a series circuit 43 of which the side of the EL element is grounded and of which the side of the inductor is connected to power supply terminals VEL and GND through diodes and switching elements Q1 and Q2, respectively. The circuit is alternately connected to the power supply terminals VEL and GND through the switching elements Q1 and Q2 to generate a driving AC voltage by utilizing series resonance characteristics of the EL element 41 and the inductor 42. 44 represents a constant power output circuit as a power supply. 45 represents a capacitor for stabilizing the voltage of the same. 46 represents a control circuit for controlling the switching elements Q1 and Q2.
While the driving frequency of the EL element 41 in the circuit shown in FIG. 4 is set by the timing at which the switching elements Q1 and Q2 are turned on and off, the driving voltage includes harmonics higher than the driving frequency because of the series resonance characteristics of the series circuit 43. The frequency f and impedance z of the series circuit 43 are in a relationship as shown in FIG. 5 and thus, when the series resonance frequency of the series circuit is represented by f1, the series resonance frequency f1 must be set higher than the driving frequency f2 of the EL element in order to suppress the flow of high frequency components to the EL element 41.
This has resulted in a problem in that the life of an EL element is reduced by harmonic components. Further, the driving efficiency has been reduced because it has not been possible to drive an EL element using a series resonance frequency.